The invention relates generally to Magnetic Resonance Imaging (MRI) systems and more specifically to a method and apparatus for reconstructing images in parallel MRI systems.
Generally, MRI is a well-known imaging technique. A conventional MRI device establishes a homogenous magnetic field, for example, along an axis of a person""s body that is to undergo MRI. This homogeneous magnetic field conditions the interior of the person""s body for imaging by aligning the nuclear spins (in atoms and molecules forming the body tissue) along the axis of the magnetic field. If the orientation of the nuclear spins is perturbed out of alignment with the magnetic field, the nuclei attempt to realign their spins with the axis of the magnetic field. Perturbation of the orientation of nuclear spins may be caused by application of radio frequency (RF) pulses tuned to the Larmor frequency. During the realignment process, the nuclei precess about the axis of the magnetic field and emit electromagnetic signals that may be detected by one or more RF detector coils placed on or about the person.
The frequency of the magnetic resonance (MR) signal emitted by a given precessing nucleus depends on the strength of the magnetic field at the nucleus"" location. As is well known in the art, it is possible to distinguish signals originating from different locations within the person""s body by applying a gradient to the magnetic field across the person""s body. For the sake of convenience, direction of this field gradient may be referred to as the left-to-right direction. Signals of a particular frequency acquired during application of the field gradient may be assumed to originate at a given position within the field gradient, and hence at a given left-to-right position within the person""s body. The application of such a field gradient is also referred to as frequency encoding.
However, the application of a field gradient does not allow for two-dimensional spatial encoding, since all nuclei at a given left-to-right position experience the same field strength, and hence emit signals of the same frequency. Accordingly, the application of a frequency-encoding gradient, by itself, does not make it possible to discern signals originating from the top versus signals originating from the bottom of the person at a given left-to-right position. Spatial encoding has been found to be possible in this second direction by application of gradients of varied strength in a perpendicular direction prior to acquisition of the signal, to thereby twist the phase of the nuclear spins by varied amounts. The application of such additional gradients is referred to as phase encoding.
Frequency-encoded data sensed by the RF detector coils after a phase encoding step is stored as a line of data in a data matrix known as the k-space matrix. Multiple phase encoding steps are performed in order to fill the multiple lines of the k-space matrix. An image may be generated from this matrix by performing a two-dimensional Fourier transformation of the matrix to convert this frequency information to spatial information representing the distribution of nuclear spins or density of nuclei of the image material.
Alternatively, spatial encoding can be performed in three dimensions by applying phase-encoding gradients in two orthogonal directions, followed by a frequency-encoding gradient in the third orthogonal direction, with signals acquired during the frequency-encoding gradient, in order to generate a three-dimensional matrix of k-space data. Three-dimensional Fourier transformation of the matrix will then convert this frequency information to spatial information representing the distribution of nuclear spins or density of nuclei within a volume of the image material.
Imaging time is largely a function of desired signal-to-noise ratio (SNR) and the speed with which the MRI device can fill the k-space matrix. In conventional MRI, the k-space matrix is filled one line at a time. Although many improvements have been made in this general area, the speed with which the k-space matrix may be filled is limited. To overcome these inherent limits, several techniques have been developed to effectively simultaneously acquire multiple lines of data for each application of a magnetic field gradient. These techniques, which may collectively be characterized as xe2x80x9cparallel imaging techniquesxe2x80x9d, use spatial information from arrays of RF detector coils to substitute for the encoding which would otherwise have to be obtained in a sequential fashion using field gradients alone. The use of multiple detectors has been shown to multiply imaging speed, without increasing gradient switching rates or RF power deposition. Parallel imaging techniques fall into one of two categories. They can fill in the omitted k-space lines prior to Fourier transformation, by constructing a weighted combination of neighboring lines acquired by the different RF detector coils. Or, they can first Fourier transform the limited k-space data to produce an aliased image from each coil, and then unfold the aliased signals by a linear transformation of the superimposed pixel values. In either case, the reconstructed images tend to suffer from incomplete removal of aliasing artifacts, especially for large speedup factors. In images corrupted by aliasing, the edges of the image are wrapped into the center of the image.
Another problem in the reconstruction of the image is degraded signal-to-noise ratio (SNR) in some regions of the image. One reason for both of the above effects is the overlapping of the sensitivity profiles of the coils in the array. Insufficiently distinct sensitivity profiles can lead to both residual aliasing and degraded regional SNR. One way to overcome the above problems is to keep the sensitivity profile of each coil orthogonal to all of the others.
It is therefore desirable to reconstruct an artifact-free image, by reducing the effects of aliasing and degraded SNR.
Briefly, in accordance with one embodiment of the invention, a reconstruction method for use in a parallel MRI system is provided. The MRI system comprises a plurality of MR detector coils arranged in an array and each coil has a corresponding spatial sensitivity profile. The method comprises detecting a plurality of MR signals from the plurality of MR coils and processing the detected MR signals with at least one filter bank to reconstruct at least one image.
In an alternate embodiment, a parallel Magnetic Resonance Imaging (MRI) system is provided for reconstruction of images. The MRI system comprises an array of magnetic resonance (MR) detector coils arranged in an array for detecting a plurality of MR signals, each of the coils having a corresponding spatial sensitivity profile and a processing means for processing the plurality of MR signals with at least one filter bank to reconstruct at least one image.